1. Field of the Invention
The present invention relates to a thin film transistor and a manufacturing method thereof, and further relates to a semiconductor device provided with a thin film transistor and a manufacturing method thereof. The present invention also relates to a liquid crystal display device, EL display device and other display devices provided with the thin film transistor, and a manufacturing method thereof.
2. Description of the Related Art
Recently, researches on high performance semiconductor devices (specifically, liquid crystal displays, EL display devices and other display devices) using a thin film transistor have been promoted. A thin film transistor (hereinafter referred to as a TFT) with high mobility needs to be realized particularly for a semiconductor device which is required to have high mobility and a high performance. As a method for improving the crystallinity of the semiconductor film, a metal element as typified by Ni which drives crystallinity is doped, formed into a film, or applied on the semiconductor film, and heated thereafter to form a crystalline semiconductor film.
Through the process of crystallization described above, a crystalline semiconductor film with large particle size is obtained, and a semiconductor device, wherein grain boundaries are connected with each other and grain defects are less, is obtained further.
However, metal elements typified by Ni have an adverse effect on device properties of a TFT and consequently; a step for removing the elements (hereinafter referred to as a gettering step) has been carried out. A gettering step is known as a technique in which metal impurity assimilated into a semiconductor segregates to a gettering site (a gettering sink) with some kind of energy, and impurity density at an active device region is reduced.
Gettering steps are divided broadly into two categories, an extrinsic gettering and an intrinsic gettering. The extrinsic gettering provides a gettering effect by applying a strain field or chemical action from the exterior. Meanwhile, the intrinsic gettering utilizes a strain field of a lattice defect caused by oxygen that is generated inside a single crystal silicon wafer.
As a specific example of gettering steps, a gettering is performed by forming an impurity region doped with rare gas elements (rare gas) and segregating a metal element contained in a semiconductor film to the impurity region by heat treatment (refer to Japanese Patent Laid-Open No. 2002-203789, for example). Further, by forming a material with tensile stress at specified elongation or more on a semiconductor over a substrate, an impurity element in the semiconductor is gettered to the material (refer to Japanese Patent Laid-Open No. 2001-223219, for example).
Thus, after passing through various steps typified by a gettering step, a crystalline semiconductor film is obtained.
However, gettering steps as described above have the disadvantage of high cost due to an increase in the number of steps. Also, a manufacturing method of a crystalline semiconductor film, which does not require a gettering step, has been in demand.